1. Field of the Invention
The present invention relates to a zoom lens which is suitably used for a TV camera, still camera, or the like and properly uses aspherical surfaces as some parts in a lens system to have a large aperture, high power, and good optical performance throughout the entire magnification range.
2. Related Background Art
Conventionally, zoom lenses having large apertures, high magnification ratios, and good optical performance have been required for a TV camera, still camera, video camera, and the like. In broadcasting color TV cameras, in particular, importance is attached to operability and mobility. In accordance with such requirements, 2/3- and 1/2-inch compact CCDs (solid-state image sensing devices) have become mainstream as photographing devices.
Since such a CCD has an almost uniform resolution throughout the entire image sensing range, a zoom lens using this device is required to have an almost uniform resolution in a range from the center of the frame to the periphery of the frame. Recently, with an increase in the density of CCDs, the resolutions of cameras have increased, and hence increasing demands have arisen for zoom lenses having higher performance.
For example, a zoom lens is required to have various aberrations, such as astigmatism, distortion, chromatic aberration of magnification, corrected, and have high optical performance throughout the frame, and a high magnification ratio. The zoom lens is also required to be compact and lightweight.
Of the zoom lenses, a so-called 4-unit zoom lens can relatively easily have a high magnification ratio and large aperture and is often used as a zoom lens for a broadcasting color TV camera. This zoom lens is made up of a first lens unit having a positive refracting power and used for focusing, a second lens unit having a negative refracting power and used for a magnifying operation, a third lens unit having a positive refracting power and used to correct variations in the image surface with a magnifying operation, and a fourth lens unit for imaging. These lens units are arranged in the order named from the object side.
In general, to attain reductions in the size and weight of the overall lens system, the lens system adopts an arrangement for increasing the refracting power of each lens unit. If, however, the negative power of the second lens unit for the magnifying operation and the positive power of the third lens unit are increased to attain reductions in the size and weight of the lens system by saving the space for the magnifying portion, a problem is posed in terms of variations in aberration upon zooming. Spherical aberration, astigmatism, and halo/coma, in particular, greatly vary, resulting in a great deterioration in optical performance. Variations in the aberrations due to zooming become more noticeable as the lens system has a higher zoom ratio and a speed of lens becomes faster. For this reason, various methods of correcting aberration variations due to a magnifying operation have been proposed.
For example, Japanese Patent Application Laid-Open No. 6-59191 discloses a 4-unit zoom lens, as a zoom lens having an f-number of about 1.6 to 1.8, a large aperture with a magnification ratio of about 18 to 40, and a high power, which is made up of a first lens unit having a fixed positive refracting power in a magnifying operation, a second lens unit having a negative refracting power which is variable in a magnifying operation, a third lens unit having a positive refracting power and used to correct variations in the imaging plane due to the magnifying operation, and a fourth lens unit having a positive refracting power and used for imaging. These lens units are arranged in the order named from the object side. In this zoom lens, an aspherical surface shaped to increase the positive refracting power is formed in the third lens unit.
Japanese Patent Application Laid-Open No. 8-82741 discloses a 4-unit zoom lens, as a zoom lens having an f-number of about 1.6, a large aperture with a magnification ratio of about 40, and a high power, which is made up of a first lens unit having a fixed positive refracting power in a magnifying operation, a second lens unit having a negative refracting power which is variable in the magnifying operation, a third lens unit having a positive refracting power and used to correct variations in the imaging plane due to a magnifying operation, and a fourth lens unit having a positive refracting power and used for imaging. These lens units are arranged in the order named from the object side. In this zoom lens, a flare-cut stop is disposed between the second and third lens units.
In a zoom lens, to obtain high optical performance throughout the entire magnification range with an f-number of about 1.5 to 1.8, a large aperture, and a magnification ratio of about 18 to 50, the refracting powers of the respective lens units, the arrangement of lenses, aberration sharing, and achromatic sharing, and the like must be properly set.
In many cases, in order to obtain, for example, high optical performance with little aberration variations throughout the entire magnification range and the entire focus range, the degree of freedom in aberration correction must be increased by increasing the number of lens elements constituting each lens unit. For this reason, when a zoom lens with a high aperture ratio and a high magnification ratio is to be realized, the number of lenses inevitably increases, resulting in an increase in the overall size of the lens system.
As the magnification ratio increases, variations in aberrations during zooming, and more specifically, variations in spherical aberration and halo/coma, increase. This make it very difficult to obtain high optical performance while reducing the overall size of the lens system throughout the magnification range from the wide-angle end to the telephoto end.
As a means for solving this problem, a zoom lens having aspherical surfaces or using a flare-cut stop has been proposed.
The arrangement disclosed in Japanese Patent Application Laid-Open No. 6-59191 is effective in reducing variations spherical aberration accompanying a magnifying operation, and more specifically, variations in spherical aberration on the telephoto side. With this arrangement, however, aberration variations near the intermediate focal length cannot be satisfactorily reduced. The technique disclosed in Japanese Patent Application Laid-Open No. 8-82741 is designed to remove aberration variations near the intermediate focal length, and more specifically, halo/coma, by using the flare-cut stop. This technique has drawbacks, e.g., requiring a complicated mechanism.
To improve the performance of a zoom lens while attaining reductions in the size and weight of the overall zoom lens, the refracting power of each lens unit and lens configuration must be properly set. In order to increase the power of a zoom lens, it is important to achieve the optimal balance between the refracting powers of a variator lens for the magnifying operation and the compensator lens for correcting variations in the image surface upon magnifying operation and the overall zoom lens system.